1. Field of the Invention
The present invention relates to a structure and design of the communication device, and more particularly, to a structure and design of the filter using multi-layer ceramic technology.
2. Description of the Related Art
Recently, the quality of communication service is getting higher due to the development of the advance wireless communication technology. Current communication products dainty not only to be light, thin, short and small, but also have good and stable communication quality. Particularly, in the wireless communication receiver, the filter is an indispensable device. The filter is mainly used for receiving the required signal and rejecting the undesired interference and noise attained by the antenna. In general, the filters are divided into lowpass filter, highpass filter, bandpass and bandstop filter according to their spectrum characteristic. Lowpass filter rejects the higher frequency signal and allows the passing of the lower frequency signal. Highpass filter rejects the lower frequency signal and allows the higher frequency signal. Bandpass filter passes the signal within a finite bandwidth and, in contrast, the bandstop filter rejects the signal within a finite bandwidth and passes the other signals spectrum.
According to the implement methods, the filters can be divided into three classes: lumped filter, distributed filter and semi-lumped filter which is combined from lumped elements with distributed circuit. In general, the distributed circuits are constructed from several transmission lines and the sizes of the transmission line are determined by the wavelength at the operating frequency. The distributed circuit operated at lower frequencies will have higher sizes, and therefore the semi-lumped or distributed filter is not suitable as operating below 200 MHz due to size limitation. However, the distributed circuit is very useful at higher microwave or millimeter-wave due to the smaller size than that of the lumped circuit. Although the lumped filter has feature of the compact size, it become impractical as operating at frequencies higher than GHz due to larger power dissipation and the parasitic effect. The lumped circuits can be much improved by the use of the superconductor recently, but such application is limited except in the universal space since the superconductor device must operate at ultra low temperature.
The ceramic filter using high dielectric constant ceramics, such as coaxial or mono-block type, are extensively used due to their features of miniaturized size and excellent performance. Such high performance filter is resulted from circular or smooth conductor geometry, and therefore the conducting loss is lower. The features of the miniaturized size are resulted from the use of the high dielectric constant material causing shorter wavelength. Such filters use the transmission line of quarter wavelength to realize the impedance transformer, addimitance transformer and resonator, and therefore the use of low dielectric constant materials are not suitable as operating between several hundred MHz and several GHz.
Recently, the low temperature co-fire ceramic technology used in the RF circuit attracts much attention. The multi-layer ceramic integrated circuits using low temperature co-fire ceramic technology have several advantages, for example, the ability of the mass product; the high three dimension integration with embedded the capacitor, inductor and resistor into multi-layer to reduce the volume; and fabrication of all passive components in one process. Since the passive components occupy above 85% area of the planar RF circuit, and therefore such circuit structure can much reduce the size of the RF circuit module.
U.S. Pat. No. 5,523,729, issued to Nakai et al, discloses similar multi-layer bandpass filter. The conventional circuit comprises input and output coupling capacitance, resonator-capacitance, resonator-coupling inductance and pole-tuning capacitance. However, there are two disadvantages in the patent. The first is that the filter performance will be effected by the adjacent circuit or the coupling effect between the devices since the part of the filter are exposed in the air. The second is that the filter is not easily integrated with other sub-module into single and miniaturized module of multi-function since the filter can not be embedded into the dielectric layer.
W. R. Smith et al. in “Design of Miniature Multilayer On-Package Integrated Image-Reject Filters,” of IEEE Trans. Microwave Theory Tech., vol. 51, No 1 January 2003 proposes a circuit structure of the multi-layer filter, enable to have a transmission zero in the rejection band. However, such filter needs larger grounding capacitance, and therefore the use of the high dielectric constant substrate is needed to reduce the size. The high dielectric constant substrate causes the finer line spacing and line width, and therefore the filter performance is sensitive to the variation of the fabrication. Moreover, the variation of inductance and capacitance used in the filter are in the fabrication process cause the shift of the transmission zero. This shift may be compensated by post-tuning process put it increases the cost of the filter.
According the above problems, the related field needs a new filter structure to overcome the disadvantage of the prior art.